The present invention relates to a system and method for tuning a process controller wherein a process model is assumed and identified during controlled self-oscillation tuning.
A Proportional, Integral, Derivative (PID) controller is a common controller used in industrial processes, including computer-controlled industrial processes. Such PID controllers and their variations and combinations (for example, P, PI, PD, PID, and so forth), have enjoyed wide-spread application in the control of industrial processes, with typical industrial processes being controlled by one or more feedback loops incorporating PID controllers.
There are well established methods, for example, the Ziegler-Nichols method, for the tuning of the parameters of PID controllers in order to optimize the controller for the process under control. While manual tuning of PID controllers is possible, it is often tedious and error prone. Moreover, the process under control may change over time, thus requiring retuning.
Several automatic tuning approaches have been proposed. For example, in U.S. Pat. No. 4,549,123, controlled self-oscillation is achieved using two state or relay control in order to calculate the Ultimate Gain, and Ultimate Period, of the process under control. Once these process parameters are derived, the Ziegler-Nichols rules, or some variation of these rules, are used to define the parameters of the PID controller used to control the process.
In more recent years, progress has been made in defining optimal PID controller settings. For example, Internal Model Control-based tuning rules have been instituted and have gained recognition. However, in order to apply the Internal Model Control rules, a mathematical model of the process under control is required. The above-mentioned self-oscillation tuning methods do not assume a process model and do not provide sufficient data for complete process identification.
U.S. Pat. No. 4,758,943, discloses a method and apparatus for automatically tuning a process controller wherein the shape of the output signal obtained from a process affected with relay feedback is used to design a model-based controller, which is other than a PID controller. This limits the applicability of this particular automatic tuning process. In addition, this procedure requires complex calculations, which render the tuning procedure inappropriate for application to tuning time limited systems.